"Let's take a walk" suggested your girlfriend.
The two of you held hands as you walked down the beach until you were out of sight of any people or houses. The tide was out and she led you below some rocks on the beach near the high tide line, hiding the two of you from view. The two of you had been here before. Your body began to tingle as you and your girlfriend began making out. You realize you felt that tingle earlier, when you were rubbing suncreen on your girlfriend's body, and yesterday after touching the mermaid mirror. It's not just arousal, but something more.
The two of you soon tumble onto the sand. Not by choice - your legs are weakening and you can no longer stand. Your girlfriend's hands feel so good on your sensitive chest. You let out a soft moan as she squeezes your... Oh no...
"Honey...why are you growing breasts?" asks your confused girlfriend.
"My hallucination last night must have been real!" you exclaim.
You hurriedly pull down your swim trunks before your merging legs can rip them. Your dick has already retreated into your body. You and your astonished girlfriend watch as your legs seemingly zipper together, top to bottom, into a tail fin. Orange scales are rapidly sprouting all over what had been your legs. Your heavy breathing causes your DD breasts to heave. You can feel your long, dark hair against your shoulders. You have turned into a mermaid for the second time. You look to your girlfriend for a reaction to your changes.